Understanding the halo populations of the Milky Way impacts upon a vast landscape of stellar, Galactic and extragalactic astrophysics. Topics likely to play important roles at this meeting are introduced, including aspects of properties of the outer halo, the halo-to-disc transition, globular cluster binary stars and dynamics, chemistry, and age determinations.

The recent results for the relation between M V and [Fe/H] for RR Lyrae variables and horizontal branch stars are reviewed: < M V(RR) > = 0.15[Fe/H] + 1.08. If [O/Fe] = 0.3, the oldest clusters' ages approach 20 Gyrs, and the the most metal-poor clusters are older than the most metal-rich by several Gyrs. If [O/Fe] ≃ −0.4[Fe/H], the most metal-poor clusters have ages of about 14 Gyrs and the most metal-rich clusters are not much younger. In both cases, there are age spreads among the intermediate metallicity clusters of up to 6 Gyrs.

The RR Lyrae and globular cluster populations are used to study the Pop II in the Galaxy and the LMC. The metallicity gradient in the Galaxy changes abruptly at the position of the solar circle. The statistics of field RR Lyraes imply that the Pop II field population is older towards the Galactic center. The density of RR Lyraes in the solar neighborhood implies a luminous mass density of 6.4 ± 1.8 × 10–5M⊙ pc–3 for the Galactic halo. The total luminosities (MV) for the Galactic and LMC halos are −18.4 and −15.1, and the ratio of globular cluster to luminous Pop II mass is 0.02 in both cases. The agreement of this ratio in two systems with very different tidal fields argues against the formation of the field population as the disruption of many smaller systems.

The central kiloparsec of the Milky Way contains a distinct stellar population that resembles distant ellipticals and bulges. An abundance range from 1/10 to nearly 10 times the solar metal abundance produces evolved stars ranging from RR Lyraes to late M giants, OH/IR stars, and Miras. I describe the kinematics, structure, chemical evolution and possible age range of the galactic bulge. Infrared imaging of the M31 bulge reveals a population of luminous AGB stars that may have progenitors younger than 15 Gyr. It is important to measure the age of bulge populations relative to the old globular clusters, and to consider a range of formation scenarios.

A new study of more than 400 K giants in the inner Galactic bulge confirms that many have high metallicity ([Fe/H] ≪ 0). Such stars are valuable templates in understanding the stellar populations of elliptical galaxies, and may help explain some of the puzzling line–strength anomalies seen in the integrated spectra of giant ellipticals.

Deep CCD photometry for a number of bulge-projected metal rich globular clusters and their nearby field background have been obtained. The V/V-I or I/V-I color-magnitude diagrams of the observed clusters are similar to the background fields and show high metallicity peculiar features. They are all very compact and slightly elongated.

The intrinsic shape and the kinematics of the Galactic bulge and the metal-poor stellar halo are discussed. There is strong evidence that the Galactic bulge is triaxial, and is rotating rapidly. The stellar halo is nearly round, rotates slowly, and has an anisotropic velocity distribution.

Results from a complete survey of proper motions to B = 22.5 at the North Galactic Pole are summarized. Evidence from this and other surveys indicates that (1) the thick disk may extend to as much as 5.5 kpc above the Galactic plane at the solar radius, and (2) the thick disk may contain stars with metallicities as low as [Fe/H] = −1.6 or lower. These two properties of the thick disk mean that surveys of halo stars risk serious contamination by thick disk stars unless very conservative selection criteria are used. Applying these conservative selection criteria to existing surveys of halo stars reveals a surprising result - namely, that the halo is in retrograde rotation.

In this review, I will concentrate on the observational aspects of the dynamical evolution of the disk population. First I will discuss some general properties of disks, such as their structure and kinematics and the thick disks. The next section is on the kinematical properties of the galactic disk near the sun, including some important new results on the age-velocity dispersion relation and the age-metallicity relation. Finally I will discuss some properties of the galactic thick disk. Two important questions, chemical gradients and dark matter in the disk, could not be included in this review.

For a sample of 189 disk stars, essentially dwarfs of spectral type F, we have determined chemical abundances of O, Na, Al, Mg, Si, Ca, Ti, Fe, Ni, Y, Zr and Ba, as well as ages and space motions. These stars have deep enough convection zones so that the surface layers may be presumed to be well mixed and thus representative of the chemical composition of the pre-stellar cloud. The sample was selected without kinematical bias such that the stars represent different metallicity groups ranging from [Fe/H]=-1.0 to 0.3 and such that ages, varying from 1 to 15 Gyears, can be derived from isochrones. Thus, the chemical composition of the interstellar medium, as a function of age and galactocentric distance (approximated by R(m) = 0.5*{R(apogal)+R(perigal)}) may be mapped. The following basic results are obtained:

1. The alpha-element (O, Mg, Si, Ca, Ti) abundances vary relative to Fe in a well defined way as a function of [Fe/H] with an excess of [alpha/Fe] for the more metal-poor stars. The excess is significantly greater for O than for the other alpha elements, which is not expected from recent SN II model calculations. There is, for the most metal-poor stars, a tendency for the alpha excess to decrease as a function of R(m). This probably indicates a more rapid star formation in the interior parts of the early Galactic disk. The individual scatter around these relations is about 0.05 dex, as expected from the observational errors, indicating an efficient mixing of gas enriched by alpha elements and Fe, respectively, in the disk at all times.

2. The abundances of the “odd” elements Na and Al, relative to Fe, show a less pronounced variation with [Fe/H], Al yet being more similar to the alpha elements in this respect. This puts further constraints on supernova models.

3. The Fe abundance relative to H shows a general tendency to decrease with increasing age. The scatter around this relation (0.2 dex) is, however, significantly greater than the expected observational differential uncertainty in [Fe/H] and is also hard to explain as a a result of bias in the selection procedure or of orbital diffusion, and even as a combination of these. This suggests that mixing of metal-rich and metal-poor gas in the Galaxy is significantly less efficient than that of SN II (alpha-rich) and SN Ia (Fe-rich) gas. This might be explained as a result of infall of metal-poor gas; however, a very efficient triggering of star-formation by this infall is then necessary if current estimates of the infall rate are typical for the history of the Galactic disk. Alternatively, one has to invoke identical or similar sites for the production of alpha elements and Fe, yet giving relative contributions of these elements that vary as a function of Galactic age. The scatter in [Fe/H] at a given age and R(m) could then be explained as the result of star formation occuring both in well mixed interstellar gas and in gas enriched by local supernovae. Another, more speculative explanation would be that the Galactic disk is the result of the merging of two different galaxy populations.

4. The abundances of Ba and other s-elements relative to Fe show a significant variation with age at a given [Fe/H], indicating a long characteristic time scale for the formation of these elements, yet different from that of the formation of Fe.

5. The three most s-element rich stars were found to have very similar R(m). We suggest an s-element rich gas cloud to be the common origin of these.

6. There is a significant tendency for some additional grouping in the dynamic-chemical space for our sample of field stars. We tentatively suggest a group of less than 10 sample stars to which the Sun belongs to be enriched in Ca and Fe, relative to other elements like Na, Mg, Al and Si, as compared with other stars of solar overall metallicity.

The details of this study will be published in forthcoming papers in Astronomy & Astrophysics.

The large reported discrepancy between the scale height in the Galactic Bulge for objects such as RR Lyrae variables on the one hand and M-giants or IRAS sources on the other is primarily due to these scale heights being determined at different mean galactocentric distances. Mira variables apparently provide a Bulge-like population in the solar vicinity and show that the thick disc consists of a number of subsystems. There are M-giants in the extended solar neighbourhood, away from the Galactic plane, which seem similar to M-giants in the Bulge. Photometric and kinematic studies on some of these stars are in progress.

We discuss observational data and dynamical considerations on the age dependence of the spatial distribution and of the velocities of stars in disk galaxies (thick disk, thin disk, spiral arms). We present new results on the age-velocity relation, on the heating of a galactic disk by massive black holes, and on theoretically predicted luminosity and colour profiles in edge-on galaxies.

I will discuss the spiral arms of our Galaxy as indicated by the Wolf-Rayet star population and their connection to GMCs, the sizes and shapes of some nearby associations in the northern Milky Way, and the slope of the IMF for Cyg OB2 and several regions of the Magellanic Clouds.

Finite mixture models provide a useful framework for analysing the overlapping spatial, kinematical, chemical, and age distributions of stellar populations. In this paper, the age-metallicity relationship is used to illustrate the properties of bivariate mixture distributions. The interpretation of bivariate scatter plots and ‘binning’ diagrams is discussed, and population membership is examined by computing posterior mixing proportions.

Models of population synthesis are resting on an ensemble of other models, data and assumptions, in particular the models of stellar evolution, the data on initial compositions, the data on stellar spectra, the initial mass function (IMF), the star formation rate (SFR), the infall rate, etc… Here we shall concentrate on the properties of stellar models and their consequences for the global properties of stellar populations in galaxies.

The basic link between star properties and population synthesis is expressed in the so-called fuel consumption theorem (cf. Renzini and Buzzoni, 1986). It says that the contribution of stars in any post main sequence stage to the total luminosity of a star population is proportional to the amount of fuel burnt in the considered evolutionary stage. The demonstration of this theorem is rather straightforward. And although it is not used in recent population synthesis (cf. Guideroni and Rocca–Volmerange, 1987; Charlot and Bruzual, 1990), its didactical value is great. It shows how the integrated properties of star populations in galaxies directly depend on stellar internal properties. Indeed, the amount of fuel burnt in a given stage depends, in turn, on many physical assumptions in the models, such as the nuclear reaction rates, the opacities, convection and overshooting, mass loss, mixing processes, etc… Thanks to the above theorem, the effect of different model assumptions on population synthesis can be predicted, at least qualitatively.

Different chemo-dynamical models dealing with the formation of stellar disks in the course of galaxy evolution are presented. One-dimensional models concerning the vertical stratification of disks are in strikingly good agreement with the solar vicinity. Two-dimensional investigations of the global evolution of disk galaxies show from preliminary results that the disk formation is delayed by means of an equatorial outflow of hot metal-enriched gas that stems from supernova ejecta in the star-forming central region.